The present invention relates generally to an interferometer system for position measurement, and more specifically, to an interferometer system and method for improving the accuracy of interferometric measurements.
Often a semiconductor exposure system used to manufacture semiconductor devices uses a laser interferometer to accurately measure relative displacement between two members. For example, the laser interferometer is used as an apparatus for measuring the coordinates of a wafer stage (i.e. substrate stage) or reticle stage to facilitate highly accurate positioning of a semiconductor wafer or reticle relative to stationary projection optics.
FIGS. 1 and 2 show a prior art laser interferometer system. The interferometer system typically measures a change in position of measurement mirrors Mx, My attached to movable stage S relative to stationary reference mirror MR attached to projection lens PL. A laser source generates beam B of light, part of which is reflected from reference mirror MR and part of which is reflected from measurement mirror Mx (FIG. 2). The light reflected from mirrors Mx, MR is then combined and reflected into sensor SR. If measurement mirror Mx moves relative to reference mirror MR, the intensity of the combined beam periodically increases and decreases as the reflected light from the two paths alternately interferes constructively and destructively. This constructive and destructive interference is caused by the two beams moving in and out of phase.
In principle, each half wavelength of movement of the measurement mirror results in a total optical path change of one wavelength and thus, one complete cycle of intensity change. The number of cycle changes indicates the number of wavelengths that the measurement mirror has moved. Therefore, by counting the number of times the intensity of the light cycles between darkest and lightest, the change in position of the measurement mirror can be estimated as an integral number of wavelengths. The simple counting of light cycles, however, does not divulge in what direction the mirrors are moving.
Actual interferometers are more complicated to be able to determine the direction of movement. This information is determined in a number of ways that may include, for example, comparing the interference pattern from light of two different wavelengths. And, while data may be processed using a central processing unit, there are also interferometers that output their data optically and therefore do not communicate with a computer in the same manner.
Interferometer design also may vary in the placement of certain functional elements. For example, in FIGS. 1 and 2, if the reference mirror MR were positioned a fixed distance from sensor SR (i.e. not on projection lens PL) and only measurement mirror Mx could move relative to sensor SR and reference mirror MR, the interferometer would be a type of xe2x80x9cabsolute interferometer.xe2x80x9d An absolute interferometer measures the movement of the measurement mirror. A differential interferometer, however, determines the relative movement between measurement mirror Mx and reference mirror MR where either mirror can move relative to sensor SR. A differential interferometer does not determine which mirror moves, just that one mirror moved in relation to the other. Therefore it does not produce enough information to determine the movement of either mirror relative to a third object without some modification. The interferometer shown in FIGS. 1 and 2 is one such differential interferometer.
As manufacturers of integrated circuits attempt to increase circuit density and reduce circuit feature size in the devices manufactured, interferometers are required to provide more precise measurement data. The precision with which interferometers provide position control has been improved by technical advances in the design of various optical components including lasers and photosensors. But the performance of interferometers is still limited by changes in optical path length due to environmental disturbances, such as thermal expansion, that cause movement of the optical components of the interferometer system. When optical components, such as beam splitter BS, tilt or rotate for any reason whatsoever the distance between reference mirror MR and beam splitter BS changes (FIG. 2). This movement of beam splitter BS causes an error in the position measurement of stage S which results in misalignment of circuit patterns on wafer W relative to one another.
Current interferometer systems fail to account for the rotation of elements within the interferometer system. In particular, see FIG. 3., where interferometers directed to reticle stage (not shown) and wafer stage S are mounted on different blocks. For the purposes of this discussion and this patent in general these blocks are considered rigid bodies and the interferometer elements mounted on them are considered rigidly mounted. But these blocks are not rigidly joined to each other and any relative movement of these blocks will cause the interferometer to produce inaccurate data for the position of wafer W relative to reticle R. Such inaccuracy leads to poor product because the image from reticle R is not in its intended position on wafer W. Current interferometer systems do not and can not compensate for relative rotation of the blocks because they do not contain the means necessary to determine what that movement is.
Precise positioning of the circuit patterns is desired to prevent an imperfect final product, a potential decrease in process yield, and a corresponding increase in manufacturing costs. There is, therefore, a need for an interferometer system with the improved capability to determine and compensate for undesired rotation of the optical components of the interferometer system.
The present invention overcomes deficiencies of the prior art by providing an interferometer measuring system that uses the interferometer system itself to compensate for movement of both the optical components and the moveable members (e.g. wafer and reticle stages) within the system. With interferometer systems that measure the stage position and attitude (rotation and tilt) of a projection exposure device, the optical components (interferometer support blocks) of the interferometer system itself tilt or rotate due to body deformation, so reticle and wafer stage position measurement errors occur. The present invention measures the position and attitude of components within the interferometer system using the interferometer system itself, and uses this to compensate for errors that occur, providing an interferometer system that can position the components of the projection exposure device with higher precision than conventional systems.
The invention next uses the known relative movement between interferometer support blocks and a reference member to adjust the positions of the other moveable members that have been measured by the optical components on both blocks. Finally, after determining the relative movement of both optical components and moveable members, the invention compensates for undesired movement of the moveable members.
The interferometer measuring system of a preferred embodiment of the present invention generally comprises, for example, in any single plane, two movable members with attached mirrors, one reference member with attached mirror, two optical support blocks, and the light sources, splitters and reference mirrors necessary for eight interferometer axes arrayed appropriately on the optical support blocks. On the first optical block, two interferometer beams are directed toward the first movable member, and two interferometer beams are directed toward the reference member. Similarly, on the second optical block, two interferometer beams are directed towards the second moveable member, and two interferometer beams are directed toward the reference member. In this preferred embodiment the movable members are a reticle stage and a wafer stage and the reference member is the projection lens. It should be noted that, since the positions of the moveable members are determined relative to the reference member, the reference member could also move and the invention would still compensate for undesired movement of the moveable members relative thereto. Thus, the reference member is also considered a moveable member.
A method employing the present invention finds the positions of a plurality of movable members relative to each other and accounts and compensates for undesired movement of the movable members and interferometer system components. This method generally comprises first sensing the change in position of two points on each of a plurality of the movable members; calculating the relative change in position of a plurality of the movable members based on the sensed positions; and compensating for undesired movement.
Where the interferometer system includes interferometers mounted on multiple blocks, relative movement between two blocks is determined by having additional steps where the interferometers from each block sense the position of a reference moveable member; then re-define the other moveable members"" positions in terms showing their position relative to the reference moveable member. The method can include a step in which a controller then compensates for undesired movement.
The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.